In a data storage system, to improve data security and availability, a plurality of backups of data may be stored. Redundant Array of Independent Disks (RAID) 1 is an example solution. RAID 1 achieves data redundancy with disk data mirroring which produces data that are backup of each other in a plurality of independent disks within a RAID group. If RAID1 is at the optimal state, namely, if each disk within the RAID group is available, RAID may access any of the disks to read data. According to the characteristics of RAID1, data from any disk within the RAID group are the same. If one disk is busy so that the original data cannot be processed timely, data may be read from a mirroring copy on another disk directly. Therefore, RAID1 may improve reading performance.